Question

1.) Chymotrypsin

a. forms an alkoxide ion in its catalytic cycle

b. has a slow phase followed by a fast phase in its action

c. has catalytic triad with serine, cystidine, and aspartic acid

d. all of the above are true

e. none of the above are true

2.) In the mechanism of serine proteases,

a. histidine becomes negatively charged by binding a proton

b. nucleophilic attack occurs by the aspartic acid side chain

c. serine becomes covalently bound to a peptide transiently

d. all of the above are true

e. none of the above are true

3.) With respect to the mechanism of action of chymotrypsin

a. it relies on formation of histidine oxide

b. it does not require zinc

c. it is inhibited by CTP

d. all of the above are true

e. none of the above are true

Answer 1

1) (a) forms an alkoxide ion in its catalytic role

• Chymotrypsin uses the side chain of serine to catalyze proteolytic cleavage. The side chain of serine (hydroxyl group) is ionized in the reaction as a result of removal of its proton by a nearby histidine, yielding an alkoxide ion.
• The enzyme's action occurs in two phases – a fast phase that occurs first and a slower phase that follows.
• A catalytic triad is a group of three amino acids (serine, histidine, aspartate) that are found in the active sites of some proteases involved in catalysis.

2) (c) serine becomes covalently bound to a peptide transiently

• Serine becomes covalently attached to one peptide when the other peptide is released. Release of the second peptide from serine requires the action of water.

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